In order to integrate network devices, for example network cameras, speakers, access points for wireless networks or communication terminals, to a greater degree in professional and private daily routines, it is important to reduce the cables required for the connection. Wireless data transmission by way of technologies such as wireless LAN, Bluetooth and DECT does away with the need for what is known as the twisted pair cable required for data transmission but a power cable is still generally needed for the voltage supply. In the standard IEEE 802.3af Power over Ethernet (PoE), energy is supplied to the network devices by way of the twisted pair cable so the power supply cable can be dispensed with. This not only impacts on ease of use for the user but in particular it reduces installation outlay and installation costs for the network operator.
Power over Ethernet is a technology for the wire-based internet, the LAN (Local Area Network) technology most widely used at present. PoE makes it possible to supply the electrical power required for operation of the individual device by way of the data cable instead of by way of power supply cables. This means that the number of cables, which have to be laid to install a network, can be minimized, resulting in lower costs, shorter downtimes, simpler maintenance and greater flexibility during installation. It is then not necessary to equip every access node in the network with an electrical installation and install additional socket outlets. The technology is particularly suitable for applications, such as telephony over the internet protocol (IP).
In the standard IEEE 802.3af a distinction is made between two basic components. The energy consumers are referred to as powered devices (PD) while the energy suppliers are referred to as power sourcing equipment (PSE).
The standard provides for transmission with a direct voltage of 48 V, the current drain in continuous operation being limited to 350 mA. The maximum supply system power is limited to 15.4 watts and the maximum power draw of the terminal after the deduction of power losses is limited to 13 watts. During a start-up phase of 100 ms duration the terminal can pick off 500 mA. Power is supplied to the network devices by way of standard cat. 3 or cat. 5 cables and standardized RJ45 connectors. The method thereby utilizes the fact that only two of the four existing core pairs are used for signal transmission in the case of 10Base-T and 100Base-TX-Ethernet.
A central aspect of the 802.3af standard is the automatic recognition of network devices. This is achieved by means of a method referred to as resistive power discovery. The energy supply device or power sourcing equipment checks the compatibility of the network device or powered device before it supplies operating power by way of the cable. To this end it injects a minimum power at periodic intervals and in this manner identifies whether the terminal has a 25 kOhm terminating resistance. If it does, energy can be supplied by way of the network. The energy supply device or power sourcing equipment then supplies the network device or powered device with low power. The network device can then use its identifier to signal which of five supported power classes it belongs to. The power sourcing equipment can then supply the network device with the required power. Such recognition takes around 1 second in total. The different classes defined in the standard 802.3af, in which the network devices can be operated, are shown in the table below.
Maximum extractionTerminatingMaximumClassUsepower (PD)resistancepower0Default0.44 to 12.95W4420 Ω ± 1% 15.4 W1Optional0.44 to 3.84W953 Ω ± 1% 4.0 W2Optional3.84 to 6.49W549 Ω ± 1% 7.0 W3Optional6.49 to 12.95W357 Ω ± 1%15.4 W4Optional—255 Ω ± 1%15.4 W
The power consumption of the network devices is important for energy economy in the power sourcing equipment. For optimum energy management it is important for the network devices to identify their power class precisely. If it is not possible to identify the precise power class during the start-up phase, the power class 0 (default) is frequently taken as relevant. The power sourcing equipment must therefore supply the maximum energy consumption of 12.95 watts per connected network device regardless of the energy consumption actually required.
Identification of the correct power class is particularly relevant in the case of what are known as modular network devices, as known from communication terminals, as these have to be categorized in different power classes with a different number of extension levels in their power consumption depending on configuration. Known methods for optimum determination of the energy to be supplied in the power sourcing equipment are proprietary protocols after basic initialization of the network devices, with which said network devices signal the required power draw. The disadvantage of this procedure is however that the network device first has to be switched to a state, in which configuration can take place on a software basis.
Administrative interventions in the power sourcing equipment are also known, in particular in the case of network devices, which report the power class 0 (default) to the power sourcing equipment. The actual power consumption is hereby notified to the power sourcing equipment, with the result that a structure-based power limit of the power sourcing equipment is not exceeded. This administrative outlay in respect of organization and cost is disadvantageous in particular in networks, in which the network devices are frequently exchanged and replaced by others.